Display result in textbox based on the input of another textbox

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Hi all, I am very new to VB.

I need to create a form in Word 2010, which requires a user to fill in the price of an item (similar to a purchase order). There are 2 text boxes, one which user enters the price of that item, whereas the other box displays an indicator depending on the price range user keys in. For instance, if user entered Price>1000, the indicator text box needs to show "A". If Price<700, then it shows "B", and Price<300, the display will be "C".

I hope someone could help me with this. Please be concise. Thank you guys.